Possible Implications of Relatively High Levels of Initial Fe-60 in Iron Meteorites for the Noncarbonaceous-Carbonaceous Meteorite Dichotomy and Solar Nebula Formation

Cook et al. found that iron meteorites have an initial abundance ratio of the short-lived isotope Fe-60 to the stable isotope Fe-56 of Fe-60/Fe-56 similar to (6.4 +/- 2.0) x 10(-7). This appears to require the injection of live Fe-60 from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of 10 times higher than would be expected to be found in the ambient interstellar medium (ISM) as a result of galactic chemical evolution. The supernova triggering and injection scenario offers a ready explanation for an elevated initial Fe-60 level, and in addition provides a physical mechanism for explaining the noncarbonaceous-carbonaceous (NC-CC) dichotomy of meteorites. The NC-CC scenario hypothesizes the solar nebula first accreted material that was enriched in supernova-derived nuclides, and then later accreted material depleted in supernova-derived nuclides. While the NC-CC dichotomy refers to stable nuclides, not short-lived isotopes like Fe-60, the SN II triggering hypothesis provides an explanation for the otherwise unexplained change in nuclides being accreted by the solar nebula. Three-dimensional hydrodynamical models of SN II shock-triggered collapse show that after triggering collapse of the presolar cloud core, the shock front sweeps away the local ISM while accelerating the resulting protostar/disk to a speed of several kilometers per second, sufficient for the protostar/disk system to encounter within similar to 1 Myr the more distant regions of a giant molecular cloud complex that might be expected to have a depleted inventory of supernova-derived nuclides.